A common technique for manufacturing spring mattresses is the “pocket technique”. This means that the springs are enclosed in pockets, so-called covering pockets, i.e. they are individually enclosed by a covering material. In this way, the springs become relatively individually flexible, so that each spring may flex separately without affecting the neighbouring springs, thus increasing the user's comfort since his weight will be distributed more evenly over the surface receiving the load.
It also known from, inter alia, WO 00/00065, EP 1048248 and U.S. Pat. No. 5,222,264 to arrange alternately in a mattress spring units-with different properties by, for instance, arranging different units in different zones of the mattress which thus-will obtain, for instance, different degrees of hardness. A problem of these prior-art mattresses is, however, that the prior-art technique can only be used for variations of the properties of the mattress in local areas, not to affect the properties of the mattress in general. Furthermore, the known techniques for making such variations are expensive and complicated to use, which also makes the final products expensive and complicated.
A general problem of mattresses is also that different parts of the user's body press down the mattress to different degrees. This implies in the case of spring mattresses that the force exerted by the mattress springs on these certain parts of the user's body is significantly greater than the force acting on other parts of the body which depress the mattress to a smaller degree. This reduces the circulation of blood in these parts and is experienced as unpleasant and less comfortable.
To cope with this problem, it is known to arrange layers with different spring properties in the mattress. Such mattresses are disclosed, for instance, in WO 98/53724 and WO 99/35081 by the same applicant. A problem with this type of mattresses, however, is that they are relatively complicated and expensive to manufacture.
Therefore there is a need for a mattress which is easier and/or less expensive to manufacture but at the same time provides good comfort in relation to conventional mattresses.